1. Field of the Invention
This invention relates to a method for preparing 3,3-dimethylbutyraldehyde by oxidation of 3,3-dimethylbutanol. In a first embodiment, 3,3-dimethylbutanol is contacted with a metal oxide. In a second embodiment, 3,3-dimethylbutanol is treated with 2,2,6,6-tetramethyl-1-piperidinyloxy, free radical and an oxidizing agent.
2. Related Background Art
Several methods are known for the preparation of 3,3-dimethylbutyraldehyde by oxidation of 3,3-dimethylbutanol.
In EP 0391652 and EP 0374952, 3,3-dimethylbutyraldehyde is produced by oxidation of 3,3-dimethylbutanol with oxalyl chloride, dimethyl sulfoxide, and triethylamine in a dichloromethane solution. This procedure is the well known Swern oxidation, which produces extremely malodorous dimethyl sulfide as a byproduct. Use of this procedure on a commercial scale requires costly and inefficient measures to prevent release of objectionable quantities of dimethyl sulfide. This is a major disadvantage.
A similar procedure is reported in Cheung, C. K. et al., Journal of Organic Chemistry, Vol. 54, p. 570 (1989). This reference describes mixing 3,3-dimethylbutanol, dimethyl sulfoxide, trifluoroacetic acid, pyridine, and dicyclohexylcarbodiimide in benzene to produce 3,3-dimethylbutyraldehyde. The major disadvantage of this procedure is that, like the Swern oxidation, it produces dimethyl sulfide as a byproduct. Another disadvantage of the procedure is that isolation of the product requires removal of the byproduct dicyclohexylurea and distillation to purify the product.
Oxidation of 3,3-dimethylbutanol has also been carried out using pyridinium chlorochromate, as described in Wiberg, K. B., et al., Journal of the American Chemical Society, Vol. 103, p. 4473 (1981). This method has the disadvantage of using an extremely toxic chromium salt which must be completely removed from the product. In addition, use of this method necessitates expensive disposal of chromium-containing waste streams from the reaction.
It is also possible to oxidize 3,3-dimethylbutanol with a catalytic amount of PdCl.sub.2 (CH.sub.3 CN).sub.2, together with triphenylphosphine and 2-bromomesitylene in N,N-dimethylformamide and water. Einhorn, J., et al., Journal of Organic Chemistry, Vol. 61, p. 2918 (1996). This procedure, however, requires a relatively expensive catalyst.
General methods are known for the oxidation of alcohols to aldehydes with either copper(II) oxide or 2,2,6,6-tetramethyl-1-piperidinyloxy, free radical.
The vapor-phase oxidation of alcohols by copper(II) oxide is described in Sheikh, M. Y., et al., Tetrahedron Letters, 1972, p. 257. Although oxidation of a number of primary alcohols to the corresponding aldehydes is described, none of these alcohols has a branched carbon skeleton as does 3,3-dimethylbutanol. No suggestion is made that the procedure is applicable to oxidation of such compounds.
Oxidation of primary and benzylic alcohols by hypochlorite in solution, catalyzed by 4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy, free radical is described in Anelli, P. L., et al., Journal of Organic Chemistry, Vol. 52, p. 2559 (1987); Anelli, P. L., et al., Organic Synthesis, Vol. 69, p. 212 (1990). These references also mention use of 2,2,6,6-tetramethyl-1-piperidinyloxy, free radical, but do not suggest that either of these oxidizing agents is suitable for oxidation of 3,3-dimethylbutanol.
Oxidation of alcohols with N-chlorosuccinimide, catalyzed by 2,2,6,6-tetramethyl-1-piperidinyloxy, free radical is reported in Einhorn, J., et al., Journal of Organic Chemistry, Vol. 61, p. 7452 (1996). No suggestion is made that this procedure is applicable to oxidation of 3,3-dimethylbutanol.
3,3-Dimethylbutyraldehyde is an intermediate that is useful in the preparation of the sweetener N-N-(3,3-dimethylbutyl)-L-.alpha.-aspartyl!-L-phenylalanine disclosed in U.S. Pat. No. 5,480,668 and U.S. Pat. No. 5,510,508. Accordingly, a method for preparing that intermediate which is both economical and specific is highly desired.